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APPARATUS AND METHOD FOR RECOVERING OFF-GASES FROM NATURAL GAS DEHYDRATOR

Abstract

An apparatus and method for reclaiming uncondensed hydrocarbons normally
exhausted to the atmosphere from a still column of a glycol dehydrator
system, and combusting the uncondensed hydrocarbons in a burner assembly
of a reboiler after the burner assembly has been ignited by fuel gas.

1. A method for recovering water and hydrocarbon gases evaporated from
glycol in a still column mounted on a reboiler having a burner assembly
with a fire-tube and an air/gas mixer at one end of the fire tube, the
air/gas mixer having a fuel inlet, a vapor inlet and an air inlet, the
method comprising: passing water vapor and hydrocarbon vapors from the
still column to a vapor condenser to condense the vapors to liquid;
passing uncondensed vapors from the vapor condenser to a condensate
separator; passing effluent from the vapor condenser and the separator to
a self-emptying container; and passing non-condensable vapors from the
condensate separator into the air/gas mixer of the burner assembly via
the vapor inlet only after the burner assembly has been ignited by fuel
gas passed through the fuel inlet and mixed with air from the air inlet.

2. The method of claim 1, wherein the step of passing non-condensable
vapors from the condensate separator into the air/gas mixer of the burner
assembly comprises controlling the passage of non-condensable vapors
using pressure of the fuel gas as a signal.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a divisional of U.S. application Ser. No.
14/515,323, filed on Oct. 15, 2014, the entire contents of which being
hereby expressly incorporated herein by reference.

BACKGROUND

[0002] A number of systems exist for dehydrating natural gas to remove
water and other liquids from natural gas. Most of these dehydration
systems involve passing the natural gas through or in contact with one of
a number of known desiccant fluids, such as glycol. For brevity, the
desiccant fluid may hereinafter be referred to as glycol, but it should
be understood that glycol is only one exemplary desiccant fluid that may
be used with such a system. The glycol essentially absorbs the water and
other liquids from the natural gas, after which, natural gas is removed
from the dehydration system to be sold, or otherwise utilized, and the
"wet" glycol is cycled through the system to be regenerated or returned
to a "dry" state in which it can be reused to dehydrate more natural gas.

[0003] The water and other liquids absorbed by the desiccant often include
an amount of off-gases containing contaminants such as volatile organic
compounds, known in the art as VOC's, and/or aromatic hydrocarbons, known
in the art as BTEX. Such off-gases may be in a gaseous state suspended in
the water or other liquids, or may be in liquid state, depending upon
temperature, pressure, and/or other conditions. These off-gases are
generally pollutants which should not be, and in many cases, may not
legally be, released into the environment. These off-gases are generally
flammable as well.

[0004] A number of attempts have been made to find methods for storing and
disposing of such off-gases to prevent them from contaminating the
environment. Storage methods may involve routing the off-gases to a tank
where they can be held for later disposal. Well sites are often in remote
locations, however, where it can be difficult, time-consuming, and
expensive to periodically retrieve the off-gases for disposal.
Additionally, storage tanks may corrode and begin to leak over time.

[0005] Disposal methods have included flares and re-boilers to burn the
off-gases, reducing them to combustion byproducts that can more safely be
released into the atmosphere. Problems remain, however, for such systems.
For example, the off-gases are often mixed in a burner assembly with fuel
gas. If the off-gases enter, and collect in, the burner assembly before
the burner assembly is properly ignited so as to cause the off-gases to
be drawn down to the tip of the burner assembly, a flash back fire may be
created upon the ignition of the burner assembly.

[0006] To this end, a need exists for a dependable system and method that
delays the delivery of the off-gases before they reach the burner
assembly until the burner assembly is ignited and brought up to speed. It
is to such system and method that the Inventive concepts disclosed herein
are directed.

BRIEF DESCRIPTION OF THE DRAWING

[0007] The drawing is a schematic illustration of an exemplary apparatus
for recovering hydrocarbon pollutants constructed in accordance with the
inventive concepts disclosed herein shown in conjunction with a portion
of a natural gas dehydration system.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0008] Before explaining at least one embodiment of the inventive concepts
disclosed herein in detail, it is to be understood that the inventive
concepts disclosed herein are not limited in their application to the
details of construction and the arrangement of the components or steps or
methodologies set forth in the following description or illustrated in
the drawings. The inventive concepts disclosed herein are capable of
other embodiments or of being practiced or carried out in various ways.
Also, it is to be understood that the phraseology and terminology
employed herein is for the purpose of description only and should not be
regarded as limiting the inventive concepts disclosed and claimed herein
in any way.

[0009] In the following detailed description of embodiments of the
inventive concepts, numerous specific details are set forth in order to
provide a more thorough understanding. However, it will be apparent to
one of ordinary skill in the art that the inventive concepts within the
disclosure may be practiced without these specific details. In other
instances, well-known features may not be described in detail to avoid
unnecessarily complicating the instant disclosure.

[0010] As used herein the notation "a-n" appended to a reference numeral
is intended as merely convenient shorthand to reference one, or more than
one, and up to infinity, of the element or feature identified by the
respective reference numeral (e.g., 100a-n). Similarly, a letter
following a reference numeral is intended to reference an embodiment of
the feature or element that may be similar, but not necessarily
identical, to a previously described element or feature bearing the same
reference numeral (e.g., 100, 100a, 100b, etc.). Such shorthand notations
are used for purposes of clarity and convenience only, and should not be
construed to limit the instant inventive concepts in any way, unless
expressly stated to the contrary.

[0011] Further, unless expressly stated to the contrary, "or" refers to an
inclusive "or" and not to an exclusive "or." For example, a condition A
or B is satisfied by anyone of the following: A is true (or present) and
B is false (or not present), A is false (or not present) and B is true
(or present), and both A and B are true (or present).

[0012] In addition, use of "a" or "an" is employed to describe elements
and components of the embodiments herein. This is done merely for
convenience and to give a general sense of the inventive concepts. This
description should be read to include one or at least one and the
singular also includes the plural unless it is obvious that it is meant
otherwise.

[0013] Finally, as used herein, any reference to "one embodiment" or "an
embodiment" means that a particular element, feature, structure, or
characteristic described in connection with the embodiment is included in
at least one embodiment. The appearances of the phrase "in one
embodiment" in various places in the specification are not all
necessarily referring to the same embodiment.

[0014] The terms "line" and "piping" as used herein refer to tubular pipes
for conducting fluids.

[0015] Referring to the drawing, an exemplary apparatus 10 for recovering
hydrocarbon pollutants constructed in accordance with the inventive
concepts disclosed herein is schematically illustrated in conjunction
with a portion of a natural gas dehydration system 12. The natural gas
dehydration system 12 may include a re-boiler 16 having an over
temperature controller 15 and a thermostatic temperature controller 17.
The reboiler 15 includes a still column 14 mounted thereon for receiving
wet glycol from a contactor tower (not shown) via a line 18. The reboiler
16 may further contain a burner assembly 19 with a fire-tube 19a and an
air/gas mixer 19b at a proximal end of the fire-tube 19a. The burner
assembly 19 is supplied with fuel gas by a line 21 having a fuel control
valve 22 and terminating in an upstanding exhaust stack 23 for heating
the glycol. Dry glycol exits the reboiler 16 via a line 24.

[0016] The still column 14 is closed, and water vapor and aromatic
hydrocarbon gases pass via a line 26 to an air cooled heat exchanger or
vapor condenser 28 where the vapor volume is reduced by condensation.

[0017] Liquids flow by gravity from the vapor condenser 28 through a drain
line 30 to a standpipe 32 which drains to a self-emptying liquid
container 34 through a check valve 35. The upper end portion of the
standpipe may include pressure relieve valve 31 and a vent valve 31
normally open. Air vapor or gas displaced by liquid entering the liquid
container 34 is vented to the upper end portion of the stand pipe 32 via
a line 36. The self-emptying liquid container 34 is fully disclosed in
U.S. Pat. No. 4,948,010, which is hereby incorporated herein by
reference. The container 34 is connected with the fuel gas line 21 via an
instrument supply line 37 so that a float (not shown) within the
container 34 opens an internal valve (also not shown) when the float is
lifted to a predetermined level by contained liquid to allow gas pressure
from the instrument supply line 37 to discharge contained liquid to
storage through a check valve 43 in a drain line 38.

[0018] Hydrocarbon vapors leaving the vapor condenser 28 are filtered by a
filter 48 interposed in the standpipe 32. Vapor and aromatic hydrocarbon
gases in the upper end portion of the standpipe 32 pass through the vent
valve 31 to a separator 39 via a line 40. A manual ball valve 67 may be
interposed in the line 40. Condensed liquids in the separator 39 drain by
gravity through a line 42 to the depending end portion of the stand pipe
32 and to the liquid container 34. The separator 39 may be provided with
a high liquid level shut down 77, which is connected to a high liquid
level shut-down reset valve 79 to prevent liquids being passed to the
burner assembly 19.

[0019] Hydrocarbons leaving the separator 39 pass through a line 44
connected to the burner assembly 19 through a three way control valve 56.
A flame arrestor 50 is interposed in the conduit 44 upstream of the three
way control valve 56. A branch line 55 extending from the three way
control valve 56 to diverts vapors under certain conditions, as presently
explained, to the exhaust stack 23. The terminal end of the branch line
55 may include an igniter 54, such as a glow plug, for igniting vapors
passed through the branch line 55.

[0020] The over temperature controller 15 is connected with the fuel gas
supply 21 upstream from the valve 22 by a line 13. During normal
operation, over temperature controller 15 supplies fuel gas to the
temperature controller 17 via line 59 and 82 to operate valves 22, 57,
47, and 56. The reset valve 79 is interposed in the line 82. Line 59
connects the line 11 to the vent valve 31 via a pilot valve 78. The pilot
valve 78 controls the passage of instrument supply pressure via a line
83, which may be fluidly connected to the line 21. A manual block and
bleed valve 69 may be interposed in line 69. In the event of reboiler
temperature exceeding a predetermined limit, the over temperature
controller 15 shuts off gas supply pressure to the thermostat temperature
controller 17, the reset valve 79, and the pilot valve 78, thus closing
the reset valve 79, the pilot valve 78, and the fuel supply valve 22,
which in turn causes the vent valve 31 to move to a position that vents
vapors to atmosphere via line 84 and causes the three way control valve
56 to move to a position that directs vapors to the exhaust stack 23 in a
manner to be discussed below.

[0021] The air/gas mixer 19b has a fuel inlet 61 connected with the fuel
line 21, a vapor inlet 62 connected to a line 58, and an air inlet 63. A
suitable burner assembly 19 is disclosed in U.S. Pat. No. 5,665,144,
which is hereby expressly incorporated herein by reference.

[0022] As discussed above, the off-gases are often mixed in the burner
assembly 19 with fuel gas. If the off-gases enter, and collect in, the
air/gas mixer 19b before the burner assembly 19 is properly ignited, a
flash back fire may be created upon the ignition of the burner assembly
19. To this end, a need exists for a dependable system and method that
delays the delivery of the off-gases to the burner assembly 19 until the
burner assembly 19 is ignited and brought up to speed.

[0023] In one embodiment, the three way control valve 56 is controlled by
the pressure of the gas in the fuel line 21 in a way that delays the
delivery of the vapors to the vapor inlet 62 until the burner assembly 19
is supplied fuel gas from the fuel line 21 at a preselected pressure and
ignited by the fuel gas passed through the fuel inlet 61 and mixed with
air from the air inlet 63. In particular, the apparatus 12 further
includes a control assembly 64 that includes a throttling pilot valve 57
fluidly connected to the fuel line 21 and a snap pilot valve 47
interposed in an instrument line 49 and fluidly connected to an actuator
of the three-way control valve 56. The throttling pilot valve 57 is
operably connected between the fuel line 21 and the snap pilot valve 47
so as to place the snap pilot valve 47 in a condition wherein the snap
pilot valve 47 directs instrument supply pressure from the instrument
line 49 to the actuator of the three way control valve 47.

[0024] In one embodiment, the three way control valve 56 is normally
positioned to direct vapors through the bypass line 55. The throttling
pilot valve 58 may begin to operate upon receiving a preselected pressure
(e.g., 4-5 psig) from the fuel line 21. Upon being actuated, the
throttling pilot valve 57 opens to allow for the passage of gas through
the throttling pilot valve 57 and interact with the snap pilot valve 47.
Upon the snap pilot valve 47 receiving a preselected pressure (e.g.,
20-30 psig), the snap pilot valve 47 snaps opens to cause the passing of
instrument supply pressure (e.g., approximately 80 psig) from the
instrument line 49 to the actuator of the three way control valve 56 so
as to operate the three way control valve 56 in a way to cause the three
way control valve 56 to direct the flow of non-condensed vapors from the
separator 39 into the air/gas mixer 19b of the burner assembly 19 when
the burner assembly 19 is ignited.

[0025] Under normal conditions, the apparatus 10 continuously operates
under a predetermined temperature controlled by the temperature
controller 17. In the event of a malfunction, such as the temperature
rising or falling to a temperature range beyond the setting of the
temperature control, the over temperature controller 15 closes thereby
shutting off instrument supply pressure to the pilot valve 78 and the
reset valve 79. As such, the vent valve 31 is caused to move to a
position that vents vapors to atmosphere via line 84, and the burner
valve 22 is caused to close so as to cause the three way control valve 56
to move so as to direct uncondensed hydrocarbon vapors to the exhaust
stack 23 via the bypass line 55. Uncondensed hydrocarbon gases diverted
to the exhaust stack are mingled with the thermal draft in the presence
of the igniter 54.

[0026] From the above description, it is clear that the inventive concepts
disclosed herein are adapted to carry out the objects and to attain the
advantages mentioned herein as well as those inherent in the inventive
concepts disclosed herein. While exemplary embodiments of the inventive
concepts disclosed herein have been described for purposes of this
disclosure, it will be understood that numerous changes may be made which
will readily suggest themselves to those skilled in the art and which are
accomplished within the scope of the inventive concepts disclosed herein
and defined by the appended claims.